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Why do we never see high-bypass turbofan engines sharing the same nacelle on large airliners and similar aircraft?
I'm talking about this sort of thing (this is the fictional airplane from Casino Royale), rather than two (or more) engines per wing in individual nacelles:

If we ignore the weird 'drop tanks' on the outer pylons, what makes this design inferior to one with individual nacelles? I can think of some highly speculative pros and cons for both layouts:
Pros for shared nacelles:

Shared nacelle and pylon potentially means less frontal and wetted area per engine, which could maybe reduce overall drag.
Overall, this scheme seems to allow the 'center of thrust' for all the engines on one wing to come closer to the centerline of the aircraft. In some designs like the A380, where a massive vertical fin is needed to maintain yaw authority in an engine(s)-out situation, it seems like this could save quite a bit of weight and drag by reducing the size of the vertical fin.

Cons for shared nacelles:

Obviously there is an increased risk that an uncontained engine failure like the one on an A380 a few years ago could cause cascading failures in its 'neighbor'. I'm not sure how to quantify this risk, but it seems significant.
Additionally, a structural failure in a pylon/nacelle (extremely rare but it does happen IIRC) will affect two engines instead of one.
Maintenance will be probably be more difficult, since you'll have to mess with two engines just to get at one.
I'd expect that due to the structural requirements of supporting two engines on one pylon, you probably wouldn't save much weight and you might actually end up with a heavier aircraft, all other things being equal.
Maybe you'd lose some efficiency in the interaction between the exhausts of both engines in a nacelle?
It might turn out that you can't fit two large high-bypass turbofans into one nacelle without choosing between flow separation issues and greatly increased frontal area.

This is all layman speculation, though. In reality, from an engineer's point of view, is the choice of one layout over the other obvious? Why or why not?

It appears that you have answered your question in itself.

I wouldn't say never. B52s have been rocking siamesed engines for quite some time: http://www.fas.org/nuke/guide/usa/bomber/b52-981974c.jpg and the B36 was doing it even earlier http://i00.i.aliimg.com/wsphoto/v3/343762004/-Alice-papermodel-Wingspan-2-meter-WWII-US-heavy-font-b-bomber-b-font-B-36.jpg EDIT: must admit, those are probably not High bypass turbofans on either bomber, though the B52 may have been upgraded over it's lifespan

@FreeMan yeah, I did consider the B52 as a counterexample, but like you say, those aren't really high-bypass. The fact that those aircraft exist, though, may suggest that the reason we don't see doubled up high-bypass turbofans is something to do with their large size and weight?

I think there will be issues with the mass flow at take off. At take-off the engines are sucking in air from a greater area than their frontal surface. If two engines are next to each other these two areas will overlap

Actually Boeing proposed to replace the 8 engines of B52 by 4. Still that option is on the table.

Slightly off topic, but the mounting of both the engines and the pods are oddly off center. It's really rather annoying now that I've seen it.

The VC-10 seems to prove your rule. http://upload.wikimedia.org/wikipedia/commons/1/1f/Vickers_VC10_from_the_rear_arp.jpg

https://en.wikipedia.org/wiki/Lockheed_JetStar" has 2 engines / pylon as well.  Again, though, those are turbojets, not high-bypass turbofans.

The nature of a high-bypass turbofan is that there are really three streams of air involved: the one going through the combustion chamber, which will be moving VERY fast on exhaust, the one going through the main fan, which will be moving fast but not as fast as the first, and the airflow around the engine.  The boundary layers between the three streams are what allow the high efficiency and relatively low noise (compared to a turbojet; one VERY fast-moving stream through and one MUCH slower stream around).  To maintain that third flow, you need a certain amount of separation from all others.

@FreeMan   Ahhhhhhhh!!!!!!!!    no! no! no! no! no!   I can't unsee that now...

Short answer
It's the vastly different flow conditions from static to cruise which demand a separate placing of high-bypass-ratio jet engines. They would produce less thrust and more drag when paired.
Why were there paired engines at all?
The early jets had their engines mounted directly beneath or in the wing, and comparisons between separately mounted and twinned engines showed a slight advantage for the latter due to lower wetted surface area and less impact on the wing.
Arado built two four-engined prototypes of their Ar-234 jet, one with separate engines (V6, see directly below) and one with paired engines (V8, further down). The V8 became the prototype of the C version of the Ar-234.

However, with the increasing airflow of high-bypass-ratio engines the interference between both will turn pairing into a disadvantage. In cruise, only the central stream tube flowing towards the engine will be ingested, and the rest will spill over the intake lip. Placing a second engine directly next to the first will block the flow of spilled air on that side and will increase spill flow on the opposite side. This will most likely cause massive separation there if the intake is not heavily modified, leading to a noticeable drag increase. Also, the now asymmetric flow in the intake would reduce the efficiency of the fan - it needs very homogenous flow over the full cross section in today's engines.
Conversely, at low speed the engine sucks in air from all around and will face competition from a second engine, such that both will not be able to ingest as much air as when mounted separately. The consequence of pairing would be reduced thrust during take-off.
The initial disadvantage of separate engines, their collective impact on wing aerodynamics, is now greatly reduced by mounting them on pylons, so they are ahead and below the wing.

How on earth did you know about these planes. Don't tell me it was one of those ... oh yeah, those Arado planes...

@BurhanKhalid: Easy. My father had a copy of William Green's Warplanes of the Third Reich, and I read it front to back, several times. Like I did with Bill Gunston's Encyclopedia of Russian Aircraft, which I bought myself.

The reduced thrust at low speeds would seem to not only affect take-off (which I suppose can be managed, if the desire to do so is there), but also on go-around, which is... am I allowed to say "harder"? The aircraft is already in a high-drag configuration then (lots of flaps, possibly spoilers and friends, landing gear down, ...) and is slowing down for landing, and then you want as much power as possible as quickly as possible to climb back out. That feels like it would, at best, require some pretty serious changes to at least current aircraft designs.

So would it be better to simply use one huge engine instead of two smaller ones?

@dalearn: Generally yes. Only when the available engines are too small will more than one be placed on each wing.

@PeterKämpf: Or when the aircraft is REALLY huge (like A380-size or bigger).

Do the new engines planned for the B-52 (still in the same paired configuration -- see https://www.thedrive.com/the-war-zone/this-is-what-the-b-52-will-look-like-with-its-new-rolls-royce-engines ) merit some treatment in this answer?  Or are those not really "high-bypass" engines?  (Maybe they merit a new question -- why were the tradeoffs worthwhile in this particular case... )

@quietflyer They are already treated, n'est-ce pas? The new engines will have less thrust at take-off and more spill drag in cruise due to their pairing, but will still be much better due to their higher thrust and much lower fuel consumption. The optimum solution for a white-sheet  design would be larger engines placed separately, but with the constraints of an existing design (ground clearance and structure), the paired arrangement is better.

A big part of this is probably related to maintenance, as you observed. Currently, engines are generally taken straight off of the pylon. If they were paired on a pylon, They would either have to be removed together (making them essentially half as reliable) or attached in a different way.
With modern jet engines, only the largest aircraft make sense to have 4 engines, because two-engine aircraft are more efficient. This means planes like the 747 and A380 would be candidates for this design. There is actually a structural benefit of placing an engine further out on the wing. When in flight, the weight of the engine helps to relieve the bending moment on the wing. As you observed, this makes engine-out control more difficult. The placement of two engines on a wing is a compromise between structure and engine-out control.
This would also affect the amount of power the plane needs. Twin-engine aircraft must have enough power to take off if an engine fails after V1. This means that they need to continue a takeoff on 50% power. Aircraft with four engines must meet the same requirement, but this means they need to continue a takeoff with 75% power. However, pairing the engines makes it much more likely that a failure in one will affect the other. This means that the plane may be required to fly on 50% power, which will make the plane even less efficient.
Another potential issue is reverse thrust. Currently, engines can use locations around the circumference of the nacelle to eject this air. With the engines combined, each one would only have part of the circumference, so this could create flow issues.
Cases of failed engines or thrust reversers would put torque on the pylon, requiring additional weight for strength.
There could be some benefit of combining engine systems, but it would be at the cost of reduced redundancy.
This would reduce the area of the outside of the nacelle, but increase the frontal area. Considering two 120-inch diameter engines, combining them by connecting straight across on top and bottom, you decrease the perimeter from 750 inches to 615 inches, but you increase the frontal area from 22600 in^2 to 25700 in^2. Bringing the profile in between the engines (like in the photo above), will decrease the frontal area, but also increase the perimeter.

Nice point the reverse thrust, I missed it!

Why not use one big reverser across the backs of both engines?

For an engine to be efficient from a propulsive point of view, there is the need to "move a high quantity of flow with the same energy". Which means, for a given core create the bigger fan possible. So, from propulsion perspective, that design is less efficient than the A380 like configuration. You can see here more information about propulsive efficiency.
There is no so much saving in wetted area, but there will be some interference between both engines in the exhaust reduction on top the efficiency. Also the junction between both engines will create a grow of boundary layer, making the junction more draggy. I can't quantify the total drag, but is not clear for me that netly the drag saving in wet surface compensates or not the increase in viscous (boundary layer) increase in drag and the propulsion integration.
An engine is basically an aspiration machine which tries to absorb the flow arround the engine, this will be less important at cruise, but at take off both engines will compete for the air around, making them to produce less thrust. Critical engine failure take off is usually a dimensioning condition for engine size... so we have less efficiency in the most critical condition. From a global airplane perspective this makes the engine more overdimensioned.
Your point about the vertical fin is correct.
Concerning the risk of uncontained engine failure, the engines are usually design to avoid this situation. I do love this video about the test made.
In terms of structure, take into accoutn that in the condition of one engine failure you will need to include yaw momentun of having one engine operationally working and the other not. So very likely the pylon will be heavier that 2 pylons (although less draggy).
Also, this configuration doesn't make any sense, is usually cheaper to include a trim tank than using external, draggy, fuel pods.
As a example that the trade-off joining engines are not really making the business just look to all blended wings airplanes, all of them are having their engines on the upper part of the airplane separated between them (like X48).
As general rule I would say "fewer engines as bigger as possible".

From the video: "Just imagine what would happen if anything threatened the integrity of one of those fuel tanks, like a catastrophic engine failure that sends shards of metal tearing through them. That's the nightmare that haunts Rolls Royce engineers and they're about to destroy an engine to make sure it never occurs." Oops. Imagine what could happen indeed. QF32 was very fortunate that it ended as well as it did.

Some of what I have read about the "re-engineing discussion" on the B-52 is not so much on the merits of updating to more modern engines (better fuel economy, spare parts availability, etc.) but on the implications of "rudder authority".
If the BUFF went from 8 engines to 4 then the implications of a loss of 1/4 or 2/4 engines on the same side would implicate that the current B-52 rudder and horizontal stabilizer might not be adequate to land the plane safely in a 3/4 or 2/4 engine situation. To wit: replacing eight engines with four might require a huge (expensive) re-engineering of the rudder and vertical stabilizer.

Joining engines has been attempted in the past in the straight-turbojet days -- the Iluyshin Il-62 and the Vickers VC-10 used this configuration, and so did the Lockheed JetStar business jet; this was done due to the poor thrust output of early turbojets.  Modern turbofans crank out enough thrust that you don't need to pair up engines -- in fact, one proposal was floated for the USAF to replace the paired turbojets on the B-52's engine pylons with single RB-211 turbofans (thus turning an eight-turbojet-engine bomber into a four-turbofan-engine one).  This was rejected due to the upfront costs; however, the fuel savings would have been quite significant due to both the ability to use half as many engines and the improved specific fuel consumption of the turbofans.
Another issue with paired engines on a pylon is safety -- LOT 5055 demonstrated this in tragic fashion when one of its Soloviev D-30 turbofans suffered an uncontained failure, causing an engine fire and severe damage to the other engine on that side; had the Il-62 used a more traditional layout, such an incident would have been much less problematic as the fire and damage pattern would have been much more confined.

Pairing engines like that makes it more likely that the failure of one engine will propagate to the other.  This has happened on, for example, the F-18 where one engine had an uncontained failure, resulting in bits of engine (I think they were turbine blades) destroying the other.
Also, when you've got wing-mounted engines, spreading them out reduces wing bending moments, which tends to reduce wing weight.


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Slow Flight for CFI/Commercial Pilot - Should the gear be up or down?
I was practicing teaching how do demonstrate slow flight in a Cessna 172RG as an instructor, and I have always just assumed that you would extend the gear for the maneuver, but when I looked at the Practical Test Standards (PTS), it doesn't specify up or down, just selected landing gear...configurations.
Does this mean that one should be able to demonstrate slow flight both ways, with the gear extended and with the gear retracted?

XI. Slow Flight, Stalls, and Spins
  ...
Task A: Maneuvering During Slow Flight (ASEL and ASES)
  ...
c. Performance of the maneuver with selected landing gear and flap
  configurations in straight-and-level flight and level turns.

Reference: FAA-S-8081-6D

"Does this mean that one should be able to demonstrate slow flight both ways" Hum... depends on how often you expect your trainees to land without gear extended, don't you think?

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Is a lapsed Advanced Ground Instructor certificate acceptable to use as proof of FOI when taking CFI initial?
I have an AGI certificate that I haven't exercised the privileges of in the preceding 24 calendar months.
When I initially got the AGI, I had planned to take my CFI almost immediately but didn't. I'm having trouble locating the regulation that shows that since I have my AGI, I don't have to re-take the FOI written to complete my CFI.
What regulation states that this is possible?

That would be 61.183(e)(1):

To be eligible for a flight instructor certificate or rating a person
must:
(e) Pass a knowledge test on the areas listed in Sec. 61.185(a)(1) of
this part, unless the applicant:
(1) Holds a flight instructor certificate or ground instructor certificate issued under this part;

I'm guessing since it doesn't say anything about it being lapsed that it's not a factor in meeting that regulation?

@Canuk - Right. Imagine if your AGI was an airplane and the requirement was "must own an airplane". Just because your airplane is out of annual right now doesn't mean you don't still own it.

Nice analogy. Thanks!

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How is the pitot system drained?
Since pitot static systems are open the outside, how does the system remain clear of water or other contaminants?

Though it's often omitted from diagrams, most pitot tubes have one or more drain holes connected to the pressure chamber, and are essentially "self-draining" (water doesn't make it into the lines):

In addition to drains on the pitot tube itself some aircraft have separate "drip chambers" and drains for the pitot (and occasionally static) system to remove condensation or rain that got past the drains (marked as 2 in the photo below):

Regarding other contaminants, at altitude the air is generally pretty clean.
The biggest problem you'll have with contaminants in the pitot tube is usually bugs (either building a nest in the opening, or smacking into it in flight). Pitot covers can be used on the ground to prevent this (and they're also used when washing or painting aircraft to prevent water or paint from getting into the system). Often some tape serves the same purpose - but you need to be sure to remove the tape or cover before flying.

+1. Nevertheless, it is possible for water to enter the system and cause problems. In these cases the system must be drained from a sump, sometimes by forcing air through the system in a maintenance shop. See stories here and here where this happened.

@dvnrrs Yup - there have been cases where the drains in the system are inadequate - in those cases unless there's a pilot-accessible drain the system needs to be opened and drained by a qualified shop, and leak-checked when reassembled.

And blocked tubes will cause havoc for the pilot not expecting it

@CGCampbell That was one of the top 5 results for "bug in pitot tube" -- It was so hilarious I just couldn't resist!

Can you go into more detail about the drip chambers and drains within the system?

@fooot There's nothing really special about the drip chambers - basically they're a small length of tubing and a self-closing valve tee'd into the line at a low point (below the instruments) where water can collect without interfering with the air flowing through the line. Not all aircraft have these drains (in fact the PA-28 series shown in that photo shipped both with and without them).

@voretaq7 I'm interested in how they get rid of the water without affecting the system pressure.

@fooot When the drain valves are closed they make an airtight seal - the drip chambers are effectively just more "space" in the line that winds up at the same pressure as the rest of the system. In the case of the Cherokee drains they're spring-loaded to close so you can't forget to close them after draining the line in preflight. (You wouldn't/shouldn't attempt to operate them in flight as that would mess with the system pressure, though I've never seen them placarded - probably because they're so inconveniently located.)

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How hard is it to fly with night vision goggles?
I was talking to a Sheriff's Department helicopter pilot and he was saying they fly around at night with night vision goggles.  I know the military does too.
What are some of the considerations that pilots flying with night vision goggles have to deal with when learning to fly with them?
How long does it take for someone to be certified/trained to fly with night vision goggles?
What is the most difficult part about it? Could civilians do it if they owned the gear?

night vision does not give depth perception

Moderately difficult if there are bridges that don't show up on NVGs...

Its a bit of a misnomer that goggles remove depth perception.  Its more accurate to say that they alter your perception and that they take a period of adjustment to become accustomed to.  However, their use can lead to extreme disorientation and vertigo and proper training should be sought before use in flight.  Terrain contrast levels, especially shadows, can greatly alter how the goggles interpret what they're looking at.  Its not uncommon for people to think they're about to die descending through a cloud layer with similar contrast to the ground.

All those answers are pretty wrong.  One eye..Can't read the instruments.. Aviation ANVS like the ANVS 9's are dual tube.  You focus them to your liking,individually for each eye!  I personally get 20/20 with them and you mount them high and simply look under them to read the instruments! I have over 7,000 hours of single pilot NVG.  They make things safer, easier, and it's just like flying during the day, only everything is green.  Things like towers and wires (powerlines) are actually MORE visible under NVG's.  It usually takes established pilots with zero NVG time only a few hours to adjust.  Nothing to it.

Hello Skyler, welcome to Aviation.SE. Can you please rephrase your answer and tune down the tone a bit? Currently it reads too much like a rant to me. I am not voting to remove it yet since it is your first post and I think it holds valuable information. Without the first sentences, less CAPS and exclamation marks it would be a much better answer.

Please read HOW TO ANSWER

Your knowledge is truly constructive, but as you know, being polite also matters!

All those answers I see only one in addition to yours.

Actually, typically, they will only use one goggle, over one eye. (In my experience.)
Having instruments should overcome most of the difficulties of flying with NVG's. Certainly better than no night vision.
The most difficult challenge is having no depth perception.
An EMS/MEDSTAR helicopter in my hometown crashed while flying with night vision goggles for that very reason; they flew into a power line I believe which they thought was farther away.
EDIT: Old post, but I thought I'd share a story that I didn't think about at the time that is relevant.
Had a friend who was in the army and told me about having to be the lead vehicle in a HMMWV column at night in a combat zone (which means no headlights) and having to drive with a night vision goggle over one eye (which means no depth perception) on a road on the side of a cliff for several hours.  The road was only about a foot wider than the vehicle. Scary stuff.

Big difficulty is that they were focussed at infinity so you couldn't read instruments. We did a lot of work to overlay HUD info graphics into the image but the resolution at the time was too low, the info took the entire view. Now they mostly use digital viewfinders that simply relay the image from an intensified or IR camera - like google-glasses

I flew the single seat light-attack A7 for the US Navy. In 1985 I was at a base out west that was a test center, and was testing software upgrades to the flight system. When I got there, the only other pilot had been talking to the squadron down the apron. They had a bunch of Cobras that were using night vision goggles at the time.
He had gotten the pilots to loan him at least 2 pairs of each of the thermal and image intensification goggles. As far as I am aware he might have been the first one doing this sort of experiment in tactical low-level attack jets in the western world. Having completed 2 cruises in the Mediterranean, and briefing for war several times, there was never mention of this sort of thing.
He had a colleague down at that squadron who was flying Harriers and they would go up in the 2 seat trainer we had. The pilot in the back was there as a safety. With some time under their belts using the goggles at altitude they started simulating tactical missions.
I went up with the night vision glasses in the back seat with him. We did an attack profile on a couple of oil tanks, and this took us through a few canyons, and finally into a pop-up delivery. I flew a bit of the mission from the back. I found it deceptively easy.
The technology back then didn't give you any peripheral vision and this produced some anxiety. When flying we always practiced keeping our "heads on a swivel," and this made that even more important. For me, the challenge was not being lulled into a sense of complacency because I felt like I could see at night now.

Would you please consider answering this one? https://aviation.stackexchange.com/q/67793/15311

Looked at the posts and there are some god answers there. Thanks for letting me know about it. I found it an interesting read.

As a rotary wing attack pilot, I will say that the difficulty of flying on goggles runs the gambit of relatively easy and intuitive to just about the scariest thing you can do in an aircraft.
Relatively easy: “high light”, single ship, recce on a cloudless night.
Scary/difficult: 0% illum, light division, over the the ocean, windless day, boats everywhere, false horizons, interloping aircraft, non-permissive environment (our lighting is cloaked), shooting rockets and gun at an island that has friendlies several hundred meters away from our target/impacts.

I've read this twice and I'm still not sure what some of it means due to lack of proper sentence structure.

I was going for a stream of consciousness but point taken. I hope you are able to understand the content a little better now.

Near as I know, it is not difficult to do, though you must receive and log ground and flight training from an authorized instructor on NVG use and receive a logbook endorsement prior to operating an aircraft as PIC using NVG equipment.
14 CFR §§61.31(k)

(k)Additional training required for night vision goggle operations.
(1) Except as provided under paragraph (k)(3) of this section, a
  person may act as pilot in command of an aircraft using night vision
  goggles only if that person receives and logs ground training from an
  authorized instructor and obtains a logbook or training record
  endorsement from an authorized instructor who certifies the person
  completed the ground training. The ground training must include the
  following subjects:
(i) Applicable portions of this chapter that relate to night vision
  goggle limitations and flight operations;
(ii) Aeromedical factors related to the use of night vision goggles,
  including how to protect night vision, how the eyes adapt to night,
  self-imposed stresses that affect night vision, effects of lighting on
  night vision, cues used to estimate distance and depth perception at
  night, and visual illusions;
(iii) Normal, abnormal, and emergency operations of night vision
  goggle equipment;
(iv) Night vision goggle performance and scene interpretation; and
(v) Night vision goggle operation flight planning, including night
  terrain interpretation and factors affecting terrain interpretation.
(2) Except as provided under paragraph (k)(3) of this section, a
  person may act as pilot in command of an aircraft using night vision
  goggles only if that person receives and logs flight training from an
  authorized instructor and obtains a logbook or training record
  endorsement from an authorized instructor who found the person
  proficient in the use of night vision goggles. The flight training
  must include the following tasks:
(i) Preflight and use of internal and external aircraft lighting
  systems for night vision goggle operations;
(ii) Preflight preparation of night vision goggles for night vision
  goggle operations;
(iii) Proper piloting techniques when using night vision goggles
  during the takeoff, climb, enroute, descent, and landing phases of
  flight; and
(iv) Normal, abnormal, and emergency flight operations using night
  vision goggles.
(3) The requirements under paragraphs (k)(1) and (2) of this section
  do not apply if a person can document satisfactory completion of any
  of the following pilot proficiency checks using night vision goggles
  in an aircraft:
(i) A pilot proficiency check on night vision goggle operations
  conducted by the U.S. Armed Forces.
(ii) A pilot proficiency check on night vision goggle operations under
  part 135 of this chapter conducted by an Examiner or Check Airman.
(iii) A pilot proficiency check on night vision goggle operations
  conducted by a night vision goggle manufacturer or authorized
  instructor, when the pilot -
(A) Is employed by a Federal, State, county, or municipal law
  enforcement agency; and
(B) Has logged at least 20 hours as pilot in command in night vision
  goggle operations.

Do you have a link to relevant CFR?

14 CFR §§61.31(k)

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text:
What is the method to find the resonance frequency of a PowerFlow exhaust?
I asked a related question about PowerFlow Exhaust systems, which produced another question for me: How do they go about finding the resonance frequency so they know how long to make the exhaust pipe?
I guess I'm interested in the technical aspects of it. What formula do they use? Do they have special equipment?
I think it's interesting that they now offer a "short stack" for some mods:

“Classic” or Short Stack? The Classic is our original design from
  1999. It is the “ideal” tuned length for this engine. Everybody loves the performance, only a mother (or an engineer) could love the looks.
  So, we came up with a compromise: The Short Stack is virtually
  indistinguishable from the stock Cessna exhaust and will give you
  about 85% - 90% of the performance gain of the Classic in a much more
  attractive installation.

Is this one of those instances of the 80-20 rule (approximately)? It seems interesting that a similar length gives you 85-90% of the gains. I'm guess then, that it's not just about the length of the new exhaust, but also the width?

I started to formulate a concise answer but it started to get out of proportions. Basically its a combination of primary (header) tube diameter and length, how you bring the primaries together (4 cyl: 4 to 1 or 4 to 2 to 1, 6 cyl: 6 to 3 to 1 or 6 to 2 to 1) and   the diameter and the length of the secondary tube. Thats about it for older type aeroplane engines, as there rarely are  mufflers. The math is not very complicated, bu there is just too much to write down right now. Optimal flow is difficult, the 85% from optimal is not that hard to accomplish. Stock exhausts are traditionally crap.

I went and checked out the answers to your previous powerflow question, and they kinda got it right, yeah, but the principle is way more complicated than just finding a resonance, since each cylinder does not have an individual exhaust pipe. I guess they were close enough to give the big idea. I'll have to get back to this later.

Why wouldn't engine manufacturers just automatically put a tuned exhaust on? The prices don't seem like they add too much to the cost.

That is weird, but that's how it is. Tuned exhaust often means more expensive materials or/and tooling (mandrel vs. crush bend) and since you get satisfactory performance (or got, as these 0-360's are archaic), why go the extra mile... small costs add up in big series.

I think there is a misconception with how these systems work.  These are 4-to-1 or 6-to-1 header systems, as used on cars, and the primary feature of a header system is that the pipes between the exhaust ports and collector are exactly the same length, which is why, on cars, they snake this way and that, because the port closest to the collector has to have a pipe that is just as long as the port farthest away.
The objective is the make sure the exhaust pulse from each cylinder reaches the collector at an evenly spaced interval to minimize interference between the pulses, and also to have the low pressure behind each pulse help pull the next pressure pulse into the collector (they are sometimes called "extractor" exhaust systems because the pulse timing creates a kind of suction effect).
The timing of each pressure pulse's arrival at the collector is the key factor.  To the extent that tuning goes on, it's to make tiny adjustments in pipe length to account for small variations in pulse timing due to different pipe contours - a pipe with more bends may need to be shortened a tiny amount to because its pulse arrives a bit late because of its more convoluted path.  In other words the tuning is the ensure the pulses are arriving at the collector evenly spaced.
From a performance standpoint, longer equal-length pipes are better than shorter equal-length pipes because the inertia of the flow stream is higher and this helps evacuate the cylinders better, enhancing the whole suction effect, kind of the inverse of the slight supercharging effect of long intake runners on the intake side.
The short stack Powerflow system gets 85-90% of the gain because it's still a 4-1 header, which is the primary feature; it's just that with shorter pipes there is a bit of efficiency loss from the shorter flow run compared to long pipes. You also save a couple pounds with less piping.

Are the primaries the same in classic and short stack? Every discontinuation in the exhaust system sends a pulse of it's own along the pipes: collector, possible mufflers and x-pipes, and the end of the whole pipe. The short stacks secondary pipe's length is such that the pulse timing is not as good as with the longer one.

Oh and this is a waaaay better answer than the ones in the previous question

Thks. I think people confuse 4 stroke headers with 2 stroke expansion chambers, where the resonance of pressure wave reflections in the chamber are what's going on. I'm not sure it's correct to say discontinuities send their own pulses, as much as the discontinuities alter the timing of the pulse.  In any case, the difference I can see between short stack and long stack is the muffler doohickey is left off but I can't tell if the primary pipes are shorter as well.If you just download the IPC for each version and look at the individual pipe PNs perhaps it'll be obvious what the differences are.

You set up a electromagnetic force driver (usually the diaphragm driver from a horn loudspeaker) in the engine end of the muffler and drive it with sine waves of varying frequency. You then put a microphone on the exhaust end and measure the loudness of the sounds exiting it.

With this you get the resonance frequency of the muffler, but what does one do with that?


text:
Are there references for the different maneuver configurations?
Since it's helpful to have specifics, lets use a 1978 Cessna 172RG as the reference (it's what I'm most interested in knowing about anyway).
Is there resources that would have the following information (for example, I know you would enter each maneuver differently).

Maneuver: Rectangular Course
MP: 15"
RPM: 2200
Gear: Down
Airspeed: 108 Knots

I'm looking for a reference for all private and commercial flight training maneuvers that would have specifics on how to enter the maneuver that would allow the maneuver to be completed without excess airspeed (or not enough). I'm not looking for how to complete the maneuver (the AFH has this information), but how to enter the manuever. Does this exist as a general reference, or is it so specific to each airplane that it couldn't exist in a general form?

My CFI initial students fill out worksheets for each maneuver.  They find the numbers that work, and demo the maneuver with those numbers.
My primary students get a similar worksheet, where I have "grayed in" sample numbers.  I demo with these suggested numbers and as the student practices, they are free to change the numbers.  For more advanced primary students, sometimes I will give them suggested numbers which are off, and let them change the numbers to what they find works for them.
Some maneuvers are highly dependent on load / DENALT and will have varying numbers.  I encourage students to adapt the suggestions to something that works for them.  It helps them understand the performance of the plane, and of themselves.
So to answer your question, there may be a CFI out there who has a little notebook which he may share with you.  Probably 25 years ago there was a series of books, which one could buy with V speeds and maneuvers for various planes.  But I have not seen those in a couple of decades.
Finally, there are all kinds of variants.  As an example, I had a CFI applicant once, who right after takeoff reduced the throttle, and flew around the pattern at final approach speed.  Good thing were were at an rural uncontrolled field!  Eventually I convinced him to fly differently for his CFI initial.  But his worksheets would often be quite different than mine.  It took him a while to get chandelles down.
So if you are a student, I would suggest that you work out your estimate of those numbers, and ask the CFI you are working with.  You could even ask him to assist you in finding the correct numbers for your plane (with both of you in it).
Finally, the actual selection of configurations and numbers is colored by the flying style of you, your instructor, or whoever one is trying to emulate.  What is right or wrong is to a large degree subjective.  Most DPEs will let the applicant select the speed they perform a maneuver at, as long as they do it safely, and consistently.  In other words, while I might think my answer is the right one, there are many others which will work.

The exact settings will vary for each type--and sometimes even within a type. That's one of the reasons pilots need to "transition" to new types: learning the critical speeds and configuration details for standard maneuvers, what sort of control inputs are needed to execute and recover properly, etc.


text:
Are turns, while under the control of a two-axis autopilot, uncoordinated?
Since the two axes of a two-axis autopilot are pitch and roll, when the autopilot executes a turn, is it uncoordinated since the autopilot is not controlling the rudder (yaw)?

In larger aircraft, the autopilot is usually still 2-axis, but the yaw axis is controlled by separate yaw damper that is simply trying to keep the ball centered (approximately; the control logic is slightly more complex). It's main purpose is to damp the Dutch roll, but as a side-effect it also coordinates turns. Since it is separate, it can be used even when hand-flying.

Absent other control factors, potentially. It is up to the pilot to maintain coordination with the rudder. However, autopilot turns typically happen at cruise speeds and at gentle bank angles, so the ball won’t get terribly far outside its cage. Even for airplanes with aileron-rudder linkage designed to automatically compensate for adverse yaw, Jan Hudec enumerates other contributors such as P-factor that add slip or skid in a turn.
That’s just for a single-axis autopilot. Your question asks about dual-axis autopilots that have another dimension in which to go “uncoordinated,” viz. pitching up too steeply for the current power setting and causing an aerodynamic stall or pitching down so as to exceed $V_{NO}$, $V_{NE}$, $V_{FE}$, etc.
Being outside sweet spot of the flight envelope is a significant factor behind why many POHs and checklists call for the autopilot to be disengaged below certain altitudes and in critical phases of flight. Cockpit automation when properly understood, used, and monitored is great for reducing pilot workload, but no mechanism should be treated as fool-proof.


text:
Is there a common template or mnemonic for a take-off briefing?
I know that a take-off briefing is a good idea so that your actions are a bit "spring-loaded" should a unexpected event occur.  Is there a common template on mnemonic out there (something like CRAFT for instrument clearances)?
For example, I was talking to a flight instructor who broke down the altitudes based on the elevation of the airport and what he would do in the event of an engine failure at or below those altitudes (example, airport elevation is 427 ft MSL, then 900 ft MSL, land straight ahead, 1400 MSL, turn back to runway...) The notable thing in this case is that he pre-computed the altitudes so he wouldn't have to think about them if something happened and had a straight-ahead, and turn back to airport plan and altitude.

Do you mean pre-takeoff vital actions, or captains briefing?

I don’t have one for emergencies, but I get HAAPY on taking the runway, Heading, Ailerons, Airspeed, Power, Pitch, Vy.

@Jamiec Captains briefing

I have never used any checklist or Mnemonic for this, but I do use it as a sneaky way to make myself look professional to my passengers while really using it to remind myself what the hell I'm actually meant to be doing.
Typically it will contain the following

What will happen in the next 5 minutes ("We'll line up on the runway when told to.... we will begin rolling..... we will lift off at about [speed]")
What will happen immediately after take off ("We will climb until X, at which point I will turn [left|right] and head towards Y")
What the intention of the flight is ("We will continue via X & Y to Z")
Any quick emergency procedures which might be necessary (Much like your instructor demonstrated, along with briefly what the passenger should do in case of this emergency.)

I do this at the hold when with an instructor, or experienced pilot in the other seat. If its an inexperienced or nervous flyer I take longer about it, usually sitting in the aircraft prior to startup.


text:
What limits the size of the aircraft?
Why is it so hard to make large aircrafts?
Boeing stopped after the 747. They haven't made a larger plane ever since.
Airbus A380 took a long time for it to be designed.

In June 1994, Airbus announced its plan to develop its own very large airliner, designated as A3XX. Airbus considered several designs, including an unusual side-by-side combination of two fuselages from its A340, the largest Airbus jet at the time.

From 1994 to 2005, nearly 11 years.
I'm not talking about the market feasibility or the cost here, but more in terms of physics and aerodynamics. We already have so many different models of airplanes. If we have working models of airplanes, how about simply scaling them? Would it work? If not, why not?
It would be great if reliable sources from the Boing 747 or the Airbus A380 development programs could be given.

I'm not talking about the market feasibility or the cost here - you might not be, but that's at least half the answer.

"how about simply scaling them?" A linear scale in aircraft size means a non-linear scale in drag/thrust required for that aircraft to fly. The other problem isn't the airplane itself, but the airports that can handle them. The A380 is very restricted as to where it can land because many airports simply can't handle an aircraft that large.


text:
Do autopilots force the aircraft to fly over certain coordinates?
As part of a project I'm doing I plotted the flight paths of about 2000 flights around the globe, I noticed over both the North Atlantic and Pacific that there is a pattern to the flight paths:

You should be able to see Europe on the right and the USA on the left. Why is there such a common pattern in the flight paths? They all seem to cross every 10 degrees longitude and 1 degree latitude. Is there any explanation for this?

Did you mean navigational system or autopilot?  I understand most people who don't work in avionics don't worry about the nuances of autopilot vs. flight director vs. navigational computers and tend to call the who thing an "autopilot."  I do it myself sometimes.

Historically, when flying over land, aircraft follow jet routes or airways (same thing, different altitude), which are pre-defined routes from one land-based navigational station (such as VOR) to another. Variations in terrain (such as mountains), location of the VOR, and other airspace considerations all affect the location of routes available to aircraft in flight.
Over water, however, there are no terrain features, navigation is not conducted primarily with VOR, and there are far fewer airspace concerns in general.  Therefore, waypoints are placed in a grid (why not?), which encourages uniformity in flight paths over time, which is what you have discovered.

Over the North Atlantic, there are no predefined low-level or high-level airways as there are over the continents. In order for aircraft to get a tailwind for eastbound flights and get out of the wind for westbound flights, Canadian and UK air traffic control set tracks based on the expected wind conditions twice a day.
The lines you are seeing are called an 'Organized Track System'; is this part of the world, they're called 'NAT' (North Atlantic) Tracks. Five to six roughly-parallel tracks will be defined and move around every day, but they will generally go between whole degrees of latitude and 10° increments of longitude.
For example, tonight's (Dec 31/13) track BRAVO ('B') will go from a intersection in the UK called...
ERAKA (at 58°N/10°W) to
60°N/20°W to
61°N/30°W to
61°N/40°W to
60°N/50°W to
MOATT (at 58.025°N/59.928°W) to
LOMTA (at 57.203°N/62.619°W).

Here's a NAT plan written in a slightly easier to read format: TADEX 55/10 55/15 55/20 56/30 56/40 55/50 OYSTR STEAM.  TADEX, OYSTR and STEAM are VOR fixes at the entry and exit of the track and 55/10, 55/15 are the grid fixes.  These are entered into the FMC literally as you see them, 55 north, 10 west and the autopilot flies from one to the next, hence the patterns you see.

It's not that the autopilot forces the airplane over certain coordinates.  The autopilot follows a programmed flight plan (that is typically entered by the pilot or the dispatcher).  There is a huge database of standard reporting points that the flight plan is made up of, which is why you see similar routes.  So in answer to your question, it's not the autopilot, but rather there are published reporting point and standard routes that most pilots program their autopilot to follow.  The autopilot is the servant, not the master :)
This helps with search and rescue (heaven forbid!) efforts if an airplane is lost, then it can give SAR a good starting point.
GPS is supposed to help create more direct routes, but total reliance on GPS-aided navigation isn't fully implemented world-wide.

As @Steve V. mentioned below, over water those standard points were just placed on a grid, which explains your 10 degrees longitude and 1 degree latitude finding.

This map should help explain it.
The navigation points are spaced out evenly, and the planes aim for those points.

Those paths are also known as "jet routes" (or "airways", depending on the altitude).  Aircraft operating on instrument flight plans along those routes are expected to follow within certain established tolerances. http://en.wikipedia.org/wiki/Airway_(aviation)

The question here concerns the patterns "over both the North Atlantic and Pacific," which is only partially explained by your answer concerning land-based navigational points.  Perhaps you could elaborate?


text:
What call sign does Marine One use when the President is not on board?
How would the pilot of Marine One identify himself to air traffic control without the President on board? Does it have an N-number? If anyone knows, I would like a specific call sign/N-number of any of the Sea Kings. In particular, how would a Sea King pilot flying alone identify themselves to air traffic control?

The Marine squadron that runs Marine 1 and the decoys is HMX-1. I believe that their callsign is 'Nighthawk'. There are several lists out there of military callsigns, but not having been in the military, I can’t tell if they are accurate. Anyway, they say that NIGHTHAWK Helo in Marine One party, or Pres not aboard
NIGHTHAWK CH-46E HMX-1, MCAF Quantico, VA (CH-53D) http://www.ominous-valve.com/callsign.txt

So if one of the Sea Kings is flying around without the president on board, they would just say something like "JFK, this is VH-3D Nighthawk"? No specific N-number, as with most aircraft?

@birdus military aircrafts have tail number but that has nothing to do with their callsign. Callsign is most time depends on operational needs. Not registration. They work in different world from us civils.

@vasin1987 So is my guess correct, then, that the pilot would say something like "JFK, VH-3D Nighthawk" to establish contact with the tower rather than "JFK, VH-3D November-12345"? I'm just trying to piece this together from the various tidbits that people are giving me here.

@birdus my guess too that this is correct.

They wouldn’t use the helicopter type after the initial call for clearance. Just something like Nighthawk 1. Just like when an airliner calls, they don’t give the type of aircraft. e.g. Southwest 2932 or United 133. Military aircraft do not have N numbers but they do have identification numbers and can use them instead of mission identifiers.

I'm curious if this question refers to the radio call sign or the transponder code.

The question is based on a false premise that some aircraft has the permanent call sign "Marine One", and may change it when not carrying the President. But actually this is the other way around: Every military aircraft uses usual military call signs, and any military aircraft can carry the President. In that case it uses a temporary "xxx One" call sign, "xxx" depending on which branch it belongs to.

Marine one is the call sign of any of the (300+) United States Marine Corps aircraft carrying the President of the United States. So, if he's not on board, the aircraft reverts back to its original radio call sign.
In case you are asking about the Marine Helicopter Squadron One, you can find the ID from helis.com. HMX-1 uses the call sign 'NIGHTHAWK' usually, but this is prone to change depending on the circumstances. From the site:

In Korea, call signs changed on a regular basis; during the Vietnam War, squadrons might use the same call sign for more than a year. Today, squadrons deploying to a combat zone,i.e. Iraq and Afghanistan, are assigned a "theater" call sign for use during that deployment only.

Maybe I'm just dense, but it's not at all clear to me what the tail numbers (radio call signs) of the Sea Kings are from that web page. And yes, I'm talking about the Sea Kings of HMX-1.

Okay. I see. So when talking to ATC, they might initiate a conversation by saying something like: "JFK,  VH-3D Nighthawk"?

@birdus They'd get a callsign assigned for the mission by their planning and operations department. Often that will be "Nighthawk" followed by their tail number, so e.g. "Nighthawk 205" for aircraft #205, but doesn't have to be. When flying in formation, you'd get "Nighthawk 205, flight of 3" if 205 leads a flight of 3 aircraft.

@jwenting Is "205" an actual tail number of any of the Sea Kings? I'm looking for actual numbers here, if those are known. Would they start their identification the same way another aircraft would, such as with "VH-3D"? So would their initiation of a conversation with a tower go something like "JFK, VH-3D Nighthawk 205"?

@birdus 205 is just an example of a possible tail number, not one I know to be used by that unit. Mentioning your type is more something for VFR aircraft that don't have a filed flightplan (the filed plan would mention your type and you'd only then mention it if the actual aircraft used were different). If you did it at all, it'd be something like "Kennedy approach, Nighthawk 205, flight of 3 VH-3s, with you 5000ft 5 miles from Ellis Island".

You seem to be confused about a couple of things. To clear everything up and reiterate some of what was said in other answers:

US military aircraft do not have "N-numbers" but they do have registration numbers (Bureau Numbers or BuNos).
Just like civil airliners, which have N-numbers but usually use callsigns and flight numbers instead, military aircraft will often identify themselves using a callsign and mission number.

When identifying using the BuNo, ATC will refer to the flights using "the service name, followed by the word 'copter,' when appropriate, and the last 5 digits of the serial number" (FAA JO 7110.65 2–4–20 6a). For example, the Sea King with the registration number 159357 would be "Marine Copter Five Niner Three Five Seven" (VM593571).
If not using their registration number as an identifier, Navy or Marine fleet and training command aircraft use "the service name and 2 letters, or a digit and a letter, followed by 2 or 3 digits" (2–4–20 6h2), for example "Marine Four Charlie Two Three Six" (VM4C236).
USAF, ANG, and "Military District of Washington priority aircraft" use "pronounceable words of 3 to 6 letters followed by a 1 to 5 digit number" (2–4–20 6h1), which is the "tactical callsign" others have alluded to. Apparently HMX-1 uses "Nighthawk," for example "Nighthawk Eight-Four" (NTHWK84 or similar; the spellings can be forced at times).
So a Marine pilot from HMX-1 who is calling ATC might identify themselves as follows:

Potomac Approach, Nighthawk Eight-Four, flight of three Sea Kings (type code S61).

The actual mission number used in the identification will be assigned by the unit's planning and ops department, as jwenting said, and may or may not have any relation to any number painted on the aircraft.

1The service prefixes for registration-number IDs are as follows: Air Force: A, Army: R, Coast Guard: C, National Guard: G, Navy: VV, Marines: VM (7110.65 2–3–5 b4a).

US military aircraft use mission callsigns, which can change with every flight, not tail numbers. Therefore, it is impossible to provide a comprehensive list for what a particular aircraft might use.
That said, squadrons often perform the same mission repeatedly, which means using the same callsign prefix, e.g. "Nighthawk" for the squadron you're asking about. That prefix is usually followed by a mission number, similar to an airline flight number.
However, some missions are unique and therefore do not need a number, and some missions (e.g. Air Force One and Marine One) always use the same number. The only hard rule is that there should never be two aircraft in the air at the same time with the same callsign.

Guard helicopters very commonly call with their BuNo instead of a mission callsign, at least around me.


text:
Can the iPad use GPS data from a Garmin aera 660?
Can the iPad use GPS data from a Garmin aera 660 portable GPS unit? I assume this would happen via Bluetooth, if it's possible. I would like the more accurate WAAS data from the Garmin for use with CloudAhoy.

I can't answer your question directly but FWIW I've never succeeded in getting my Garmin GDL39 to talk to Cloudahoy. From what little I could find out, Garmin doesn't make it easy for third-party software to use their devices.

NMEA out is supported via USB by the Aera 660. I could find no documentation of this ability via bluetooth.

The following formats are supported for connection of external devices: NMEA 0180, 0182, 0183 (versions 1.5, 2.0, 2.3, 3.01) pg173

A wireless process is Aera 660 to Garmin Pilot for logging and then to CloudAhoy.

Using Bluetooth® connectivity via Garmin Connext technology, the aera 660 easily interfaces with compatible devices to cut the clutter in your cockpit and improve your flying experience...
Using your device’s built-in GPS receiver, optional GDL 39, GLO™ for Aviation or Connext compatible configuration¹, Garmin Pilot provides full en route navigation on the rich, interactive moving map...
Uploading the automatically created track log from Garmin Pilot into CloudAhoy is a convenient alternative to logging the flight with the CloudAhoy app...

Most bluetooth external GPS devices conform to NMEA and are compatible with CloudAhoy. I don't know if that's the case with Garmin Aera 660.
However, if used with Garmin Pilot, then you can always export GP's track log to CloudAhoy. (I'm associated with Cloudahoy)


text:
Vancouver FIR meaning
In looking at Garmin Pilot, there is something labeled "Vancouver FIR" which has 3 segments. It's located over the northern Cascades and to the east of the northern tip of Vancouver Island (Canada). What is that "Vancouver FIR"?
Specifically, I'm interested in flying over those mountains at a few thousand feet AGL (in and among the mountains) and am wondering if this "Vancouver FIR" is something I need to worry about.
I'm currently working on obtaining my Private Pilot Certificate in the United States.

FIR stands for Flight Information Region.  There is lots of information about these on-line, to the point that it isn't worth regurgitating it here.
However, the FIR should be the least of your concerns at this stage of your training...
If you want to fly "in and among the mountains" of the extremely rugged North Cascades, I would strongly recommend you get your Private Pilot Certificate first, then take a mountain flying course as a minimum before you get any lower than 1000' above the highest peak in a sector, and/or gliding distance from a wide flat valley floor with suitable emergency landing spots.
Narrow valleys and downdrafts on the lee side of ridges are killers.
I live up here in Washington state and have flown in the mountains for years, but they are no place for a beginner on a student solo.  Especially this time of year.

Vancouver Center/FIR is the rough equivalent of Seattle Center, with slight differences since Canada (like most of the world) is better about following ICAO rules than the US is. One of those differences is actually naming and labeling the Flight Information Regions correctly on their maps.
How to legally fly to/in/from Canada is a much larger topic, and the slight differences in ATC are trivial in comparison to all the other things you'll need to know and do. Save learning about that for after you pass your PPL checkride, though.

What is incorrect about how the Vancouver FIR is labeled on the sectional?  FYI my Canadian chart does not show a corresponding Seattle FIR.

@MichaelHall The issue is on the US side, where FIRs are not labeled as FIRs.

can you provide a specific example?  Because I am looking at the North Cascades area the OP was asking about, and on the VFR sectional, IFR high and IFR low the Vancouver FIR is clearly marked.

@MichaelHall Vancouver FIR is on the Canadian side. Seattle FIR is not labeled, at least on US maps.

Ok, but why does that matter if you are already on the US side?  A FIR is only of concern if you are outside of it and want to either avoid it, or come in.


text:
Why ground a plane while fueling?
Why does a plane need to be grounded while fueling, but we never ground our cars? Does the potential for a spark magically exist for an airborne vehicle but not for an earth bound one?

There are cars that have conductive rubber like strips hanging down their rear axle to dissipate static and you are discouraged to sit in your car while fueling because of static and there are indeed accidents, though these days systems that don't suck in the gases are a rarity in developed countries

They ground the trucks that refills the gas station pumps.

Race cars are also grounded. So it may be more a matter of the filling nozzle and vapor recovery than of vehicle type

Many maintenance procedures are best done grounded.  It's hard to convince the fuel truck to drive that fast!

On cars I think it's because static charges don't build up as much but theoretically it's still possible for a spark to occur when the nozzle touches the tank inlet and provides a ground path.
On airplanes there are two issues:
Static charges built up in the airframe in flight.  For this it is essential to ground the aircraft before the fuel nozzle is brought to the tank inlet.  The risk is of a spark just before the nozzle touches the filler neck, the nozzle itself being grounded, which is right at the location where the air fuel ratio is favourable to ignition. It's also a good idea to touch the nozzle to the fuel cap before you remove it in case the tank isn't adequately bonded to the rest of the airframe, but I don't think anybody does this.
The other one is static charges built up within the fuel tank by the shearing action of the incoming fuel column when adding fuel.  In a large refueling operation, the column of fuel builds huge static charges within the mass of fuel itself, which result in static discharges within the fuel (there are films that show this; a freakin' light show).
Most dangerous is fueling from a plastic can.  When I used to fuel my airplane from cans I made a grounding wire arrangement that bonded the fuel in the can, the fuel tank flange and the ground (using a nail on the end of the wire pushed into the dirt) together.

Unfortunately your "nail in the dirt" didn't really do anything grounding-wise, unless the ground was extremely wet. Grounding rods are typically driven 8+ feet into the ground to be an effective "Earth". The nail was also probably galvanized iron which is a relatively poor conductor compared to copper (somewhere between 12-17% whereas copper is 100%).

Good thing for me I don't do it any more! :0

They modified the electrical code to 8 foot, because 6-foot ground rods weren't doing the job. They also require two ground rods some distance apart (unless a complicated test confirms that one ground rod is doing the job), the test costs more than a second ground rod.    The best ground is actually an Ufer, which ties into the reinforcing rod in the basement/foundation/pavement, but you must commit to this at pour time. Most people forget.

@RonBeyer: Galvanizing rods are intended to provide a safety ground in the presence of mains power. That will happily produce a sustained current in excess of 20A. Static electricity is not a sustained power source. in fact, you don't want a too good grounding. Since the charge is limited, a higher resistance will lower the peak discharge current.

We do ground cars during fueling. The biggest difference is probably just in the way the grounding is done.
Fueling Cars
Every car I've ever fueled in my life has a spring-loaded metal plate that touches the nozzle as the nozzle first begins to enter the fuel tank, before fueling would start. Any sudden static discharge would happen then. That metal remains in contact with the nozzle throughout the fueling process, keeping the car (or at least the part of it around the fuel) grounded throughout the entire fueling operation.
Fueling Light Airplanes
On the light airplanes I've fueled (Cherokees and the like,) there's a relatively large hole in the top of the wing through which the aircraft is fueled. There's no spring-loaded metal plate or anything like that. Just a hole that is significantly larger than the fuel nozzle with a cap that is unscrewed and completely removed before insertion of the nozzle. It would, thus, be not only possible, but rather likely that contact would neither be made before fueling begins, nor maintained consistently throughout fueling.
Furthermore, even if contact is made before fueling begins, the fuel tank (which hopefully still has some fuel in it) is directly beneath the hole, so any spark that did form at first contact would already be dangerous. With a car, that spark would form outside the fuel tank and would not enter the fuel tank. On the light airplanes I've fueled, it would happen at the top of the fuel tank and fly into it, a much bigger problem.
So, in order to prevent sparks falling into the fuel tanks on a light airplane, we attach a separate cable to the airplane first (away from the fuel tank) that grounds the entire (conveniently conductive) frame before we stick anything into the fuel tank that might have otherwise caused a spark.

Re "Every car I've ever fueled in my life has a spring-loaded metal plate...": You must have fueled only relatively new cars, then.

@jamesqf If by "relatively new," you mean relative to the airplanes I've fueled, yes, the cars were relatively new. :) Most of the cars I've fueled have been mid-1980s models or later, though with a bit of 70s mixed in a while back. On the other hand, the newest airplane I've fueled was built in 1980 with some dating back to the 60s.

My 1985 Supra does not benefit from the spring-loaded cover in the fuel filler, just a wide-open hole into the tank, and it does occasionally make me nervous that I'm about to touch two metal objects together in the presence of flammable vapour :) What also likely makes a difference is the filling rate - aircraft generally transfer huge volumes of fuel at a time, typically at a very fast rate while passengers are boarding, much faster than car filling. The fluid velocity in the pipes probably makes static more likely, and you want to take absolutely no chances with passengers onboard.

@Gargravarr: Re "aircraft generally...", that's one of the (IMHO) problems with this site, that questions about commercial aviation often get mixed up with questions about general aviation.  For the planes most of us are likely to own or fuel (unless we are working as ground crew at a commercial airport) the amounts of fuel, filling rates, number of passengers, and so on are not significantly different from cars.

@jamesqf In the US, they've been required since the late 1970s, as part of EPA evaporative emissions fuel tank vapor recovery regulations. Other regions with less sun and smog (Europe) came later.

@Gargravarr The OP of this question specified (in a comment) that he was asking about light GA airplanes, not jet airliners, so my answer is written in the context of light GA. The fuel filling rate and quantity is not very different from a car. The PA-28s I've fueled have 2x 25 U.S. gal fuel tanks (one in each wing.) 25 U.S. gal is similar to the fuel capacity of larger SUVs and minivans. Light GA aircraft are filled with a pump resembling the ones you'd find at normal automobile gas stations.

According to reddit,

the amount of fuel transferred when refuelling a car is generally too small to generate a dangerous amount of static charge,

and

you actually do ground your car when you fill it, the action of touching the nozzle to the filler hole grounds it.

EDIT: Also, it's not "magically", but a plane can indeed build up a considerable charge while in the air, from flying through electrically-charged clouds and whatnot - something that is not usually an issue with groundbound vehicles (such as most cars).

"you actually do ground your car when you fill it, the action of touching the nozzle to the filler hole grounds it" So then the action of touching the nozzle to the filler on the airplane grounds it, too. Still seems inconsistent.

Aren’t they using tank trucks to refuel aircraft? Those are usually on rubber tires, so no grounding from simply connecting them to the aircraft.

Not sure whether grounding through the nozzle was a good idea. If there was considerable potential between them, touching them hole could create a spark right at that location where we absolutely do not want it. That's why both the nozzle and the filler hole are grounded before coming close to each other.

@Michael I'm really more interested in light planes (e.g., a Cessna 172 or Piper Cub), although I didn't state that in the question. These are typically refueled by the owner at a gas pump not unlike the one at your local Arco.

Reddit is wrong.  These days typical SUV or oversized pickup will take as much or more fuel than my Cherokee - 50 gal max, ~40 for the usual "fill to the tabs".  Also, at least in places with serious air quality regs, the automotive gas pump nozzle is surrounded by a non-conductive rubber boot, so the metal may never touch the car.

@Bergi: Well, theoretically you could have a defined resistance (e.g. 1MΩ) between nozzle and ground to slowly drain the charge in order to avoid a spark. ESD grounding equipment for handling electronics usually works this way.

@jamesqf: I was assuming that birdus was talking about refuelling rather larger aircraft...

Possibly a pedantic point, but it isn't "according to reddit".  It is according to a particular reddit user.


text:
Gear reduction on large turbofans
Do engines like the GE90 and Trent 1000 use transmissions (planetary gears?) for RPM reduction on the fans, or are they welded directly to the same shaft as the low pressure turbine?

To add to what @ymb1 said, and to expand on your question about "welded", @birdus:
The NTSB accident dockets for Southwest 1380 / NTSB# DCA18MA142, Southwest 3472 / NTSB# DCA16FA217, and American 383 / NTSB# DCA17FA021 all show some great internals about turbofan engines and (mostly) what happens when things fail.
Specifically, Page 15 of the NTSB's Powerplant Group Chairman's Factual Report for SWA1380 has a nice description of the blades and how they attach. It's not 100% clear but reading carefully, it does describe how the blades are attached to the fan disk which is then "bolted to the booster by 24 bolts" (which it further clarifies that, at least on the CFM56, the LPC Low Pressure Compressor is sometimes referred to as the "booster")...
That said, the CFM56 and GE90 do share a common characteristic: the LPC (Low Pressure Compressor - aka "N1") actually has compressor stages attached to the same shaft as the fan.
The Trent 1000 is pretty different - the entire Trent series really evolved from the venerable Rolls Royce RB211 and ever since then the entire series has been a triple-spool turbofan engine (as opposed to the twin-spools utilized by every(?) other manufacturer.)
So on a RB211/Trent XXXX, you have 3 independent spools and 3 gauges in the cockpit: N1, N2, and N3 - corresponding to the LP, IP, and HP shafts (low/intermediate/high). The whole idea of this being to decouple all of the core compressor stages from the fan rotor. If you look at the various compressor stage/turbine stage configurations, the notable difference with a Rolls Royce RB211/Trent is that the Fan has its own LP turbine.
The Trent 1000 it has "Compressor: one-stage LP (fan), eight-stage IP, six-stage HP compressor" and "Single-stage HP turbine (13391 RPM), single-stage IP turbine (8937 RPM), six-stage LP turbine (2683 RPM)". So that means that there's a 6-stage turbine at the back that then spins the one-stage big fan at the front.
The entire purpose of the three spools - even though it is far more complicated than having 2 - is to allow the fan at the front to rotate at a different speed than the engine core compressor sections. Doing so allows you to run the core faster/hotter without spinning the fan at the front too fast.
The PW1100G from Pratt & Whitney is their answer to the problem of needing to turn the fan at the front at different speeds. Instead of using 3 spools to accomplish the task, they use a gearbox. This allows for different fan blade geometry and allows for a much quieter engine. (Speaking from personal experience, the PW1100G on the A220/CS series is seriously quiet.)
Not to be left behind, Rolls Royce is developing the "UltraFan" which will apparently vary the pitch of the fan blades during flight.

It's debatable whether three rotors are more or less complex than two. A three shaft design is shorter, the rotordynamics are better behaved, and two independent speeds for the core compressor improves stall and surge margin. I remember a meeting between Rolls Royce and the FAA, when RR had the only three-shaft engine design, on the subject of in-flight engine shutdowns. The FAA's starting position was "RR must be mis-reporting the data, because you claim no failures in several reporting categories, unlike every other engine type." Wrong - those failures never occurred with a 3 shaft design.


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What are the points 1864 and 1922 on this in-flight map?

This was taken in a Emirates flight Dubai-Paris in December 2015.

What are on these 2 locations?
Why these numbers? Are they dates?

A picture of the map would help as I have no idea what you are talking about.

@SMSvonderTann sorry it took me a while to find a way to compress the image < 2Mb

It's pretty clear what he's asking. Shouldn't it be closed as offtopic instead?

@oldmud0 The closed vote were raised before picture uploaded

better suited for Travel SE?

Unclear what I am asking, really? Please read my 2 questions which are clearly defined in 2 separate bullet points. Excess of zeal from moderators is a typical issue on SE. The only thing which is a unclear is why they are flagging this function: is it only to gain points?

The number may stands for the year of the ships sunk.
1864 may correspond to CSS Alabama (Arabic: ألاباما), a Confederate States Navy Screw sloop which was sunk on June 1864 at the Battle of Cherbourg. The wreck is at 49°45′9″N, 1°41′42″W.
1922 may correspond to SS Egypt (Arabic: مصر), a British ocean liner which was sunk after a collision with SS Seine on 20 May 1922. The wreck is located near 48°10′N, 5°29′W.

As @mins mention in comment, there are other wrecks appearing on the map in IFE, including Mary Rose 1545 near Portsmouth and San Pedro de Alcantara 1786 near Peniche.

Interesting! Do you have any idea why this would appear on the customer screen of a commercial flight?

@RockScience I don't know. but the map services is usually contract out, not provided by the airlines, and sometimes not even the inflight entertainment system providers themselves.

Out of interest how did you get to this great answer? Did you translate the Arabic, google and cross-correlate the two numbers, or just have a brainwave?

@RockScience intuition

Related on Airliners.net.


text:
Can I fly "into" the Grand Canyon?
I haven't looked at the Grand Canyon on a sectional yet, but I know that it is definitely big enough (by far) so that someone could fly a plane under the level of the rim and stil meet all of clearance requirements for VFR flight.
Is it legal, and safe to take an airplane into the canyon?

anyone know the the minimum safe altitude for the area, my guess it is well above the rim

Anecdote alert: Having learned to fly in SoCal it was discussed, and according to a few people its restricted along its length - the reason given was to protect the livelihood and safety of pleasure flights lest any private pilot within a day's outing range go for a pleasure cruise

Apparently it's legal if you're the President (or, at least, you can make it legal in that case.) A tour guide on a tour I took there a little over a year ago recalled seeing one of the VC-25s (yes, a 747) flying through the canyon when a President was visiting. IIRC, it was Clinton.

The answer is: no, normal flights are not allowed under the canyon rim.
If you look at the sectional chart, you see this notice:

Searching through the CFR (Title 14, Part 91) brings up this Special Federal Aviation Regulation No. 50-2 - Special Flight Rules in the Vicinity of the Grand Canyon National Park, AZ.
Ther is also Supbart U of Part 93.
The Subpart U regulations seem to be the most current applicable regulations (please correct me if I am wrong).
The rule applies to everything under 18,000 feet MSL within an area around the park (like the chart says).

Except in an emergency or if otherwise necessary for safety of flight, or unless otherwise authorized by the Flight Standards District Office for a purpose listed in 93.309, no person may operate an aircraft in the Special Flight Rules Area within the following flight-free zones:

It then describes the four zones: Desert View, Bright Angel, Toroweap/Shinumo, Sanup Flight-free Zone. You can't fly through these, but between them are "corridors" in which you are allowed to fly above a certain altitude. See this map for locations of the Flight-Free Zones and corridors.
There are also minimum sector altitudes for different areas, including the corridors, which are going to be the main thing preventing you from flying very low. The altitudes are different for commercial air tours versus transient and general aviation.
Also:

no person may operate an aircraft within 500 feet of any terrain or structure located between the north and south rims of the Grand Canyon.

There are also noise limits depending on the aircraft type.
For some info on the background of these rules, see this NPS study.

Your link gives the altitude limit as 14,500 ft: Quote That airspace extending upward from the surface up to but not including 14,500 feet MSL. What is right, 14,500 or 18,000 ft?

The linked map has the answer: Flights below 18,000 ft are restricted and the excluded areas (flight free zones) end at 14,500 ft. Also, seasonally the exclusion altitude can be raised to 18,000 ft (permanently, if the National Park Service has its way).

I think the 18,000' notation basically means: "if you're going to be below 18,000', this chart is not for you; get the Grand Canyon chart which has all the details.

It is generally not legal to fly into the Grand Canyon - there is a SFRA (Special Flight Rules Area) over much of the canyon which is designed to keep General Aviation traffic from annoying tourists (and away from the commercial air tour operators). For both GA and commercial tours there are altitude restrictions which keep flights well above the canyon rim.
(You can download a chart of the Grand Canyon SFRA from the FAA)
Were it legal it probably would not be the safest idea anyway: Inside the canyon there are strong, often unpredictable winds. The conditions would be similar to flying a mountain pass, but with substantially less clearance between your wings and the canyon wall in some places.

Safe maybe, but not legal. Not anymore - the helicopter tours did fly into the canyon years ago, but it became illegal in 1987, prompted by a mid-air collision in 1986. A Special Federal Aviation Regulations (SFAR) was adopted which regulates traffic below 14,500 ft MSL. This height limit can be raised seasonally to 18.000 ft, and only commercial tour operators are allowed to fly below. For GA traffic three small north-south lanes allow crossing at 10,500 (southbound) rsp. 11,500 ft (northbound). Now even the number of fixed wing and helicopter tour flights is restricted to reduce noise.

"the helicopter tours did fly into the canyon years ago, but it became illegal in 1987, prompted by a mid-air collision in 1986. A Special Federal Aviation Regulations (SFAR) was adopted which regulates traffic below 14,500 ft MSL. This height limit can be raised seasonally to 18.000 ft, and only commercial tour operators are allowed to fly below." -- there seem to be some contradictions in these comments.  Can the commercial helicopter  tour operators legally fly below the rim, or not?  I watched it happen very often while working on the rim in 1999 and 2000.

December 1979. Air Force F4C, call sign taste 27 got permission from Albuquerque center to fly into the canyon. Was told we were making history, that we were the last jet to be allowed into the canyon.

Welcome to Av.SE! That's pretty cool - were you front seat or back seat on that flight?


text:
What precautions should I take when flying over a mountain?
I would like to go on a photo flight over Mt. St. Helens sometime while I'm in the Portland area.  I am aware of the weather conditions that are caused by mountains - but what precautions should I take to make sure that my flight over the "summit" is as safe as possible.
I think it's worthwhile to mention that there is a wilderness area over Mt. St. Helens, so I will be above 2000ft AGL.

And the type of aircraft is?

@Iceman It's a 172s, but after reading D_S' answer I think I'll read up on it a bit more, and see if it's practical in a 172.  If not I'll find a different plane :)

When flying over mountains Pilots will have to encounter unpredictable winds, high density altitude, steeply rising terrain and box canyons, mountain waves, rotor clouds, turbulence, and clouds covering high ground .
http://www.swaviator.com/html/issueAS99/basics.html
It is therefore essential to take the below mentioned factors into consideration :
(1) ROUTE PLANNING : When flying over mountains Route Planning is extremely important because a straight line between two points may not always be the best way to go . Therefore, pick a route that avoids the rugged areas and highest peaks where an emergency landing could not be made. It usually takes very little extra time to bypass the most mountainous areas and follow major roads through more populated areas and lower terrain .
Do not approach a ridge until you have sufficient altitude to cross with a safety margin. If you encounter too much sink, you must have enough altitude to turn away towards lower terrain. To make that turn as easy as possible, its wise to turn your aircraft to a 45-degree angle to the ridge before you cross .
http://www.avweb.com/news/airman/190015-1.html?redirected=1
(2) THE TERRAIN FACTOR : The rising terrain will often out climb non-turbocharged aircraft. If your aircraft has a service ceiling of 14,500 ft., that is based on standard atmospheric temperature which is 59 °F at sea level and decreases an average of 3.5 °F per thousand feet. If the surface temperature at 5000 ft. is 100 degrees, the density altitude on the ground is about 9,000 ft. When you climb to an indicated altitude of 9,500 feet you have reached the service ceiling of you aircraft.
(3) THE WIND FACTOR : In a light aircraft, it is best to stay on the ground if the winds are greater than 35 miles per hour. Due to the venturi effect, winds will be much greater in the vicinity of mountain passes. Cross passes as high as possible, as downdrafts up to 2,000 feet per minute can occur on the leeward side.When strong winds of 40 to 50 miles per hour are blowing perpendicular to a mountain range, expect mountain waves and strong downdrafts for many miles on the leeward side.
(4) THE CLOUD FACTOR : The combination of rising ground and lowering ceiling can be deadly. Mountains disappear not only in clouds, but also at night. You should not fly over high territory after sunset unless you are at the minimum en route IFR altitude which provides a 2,000 ft. clearance along airways. When flying after dark, closely monitor your altitude and when in a turn, your rate of descent.
A mountain checkout flight is all that is required to get started. The checkout will enable you to operate around some pretty impressive terrain :
Flatlander Focus : Two good books on the market are The Mountain Flying Bible by Sparky Imeson, and Flying the Mountains: A Training Manual for Flying Single-Engine Aircraft by Fletcher Fairchild Anderson. Reading either one will give you a solid foundation on which to build your skills.
Most Importantly - Know Your Aircraft & Its Limitations
http://www.caa.govt.nz/safety_info/GAPs/Mountain_Flying.pdf
(a) What distance over the ground does your
aircraft cover in a rate-one turn?
(b) What’s the distance for a steep turn?
(c) What speed and configuration do you use for reduced
visibility and for a minimum radius turn?
(d) What is the best-angle-of-climb speed?
(e) Do you know the manoeuvring speed (VA) and
understand its application in potentially
turbulent conditions?
(f) How is your aircraft performance and handling affected by
altitude and turbulence?
(g) How is performance affected when operating at
maximum-all-up-weight?
Needless to say recent practice and currency are vitally important.

@ Keegan McCarthy - Cheers ...and have a safe flight .

In addition to the above factors, you should also be mindful of possible altimeter errors, due to wind, see this related question and/or due to significant temperature variations from (colder than) the ISA model both of which cause the altimeter to over-read (indicate higher than actual) compromising vertical clearance.

Mt. St. Helens has one hazard you are unlikely to encounter with any other mountain: it's an active volcano, with the most recent eruptive activity only six years ago.  This means that on the day of your flight, you should check both the relevant NOTAMs (for closures due to volcanic hazards) and the Cascade Volcano Observatory for unusual activity.  St. Helens will usually give weeks of warning before any eruptive activity, but you don't want to find yourself above the mountain with an ash cloud rushing towards you.


text:
How many kills did Neil Armstrong have?
I realize this is slightly off topic, but there's no other SE site that it would at all fit into.
Anyway, I was just thinking about Neil Armstrong, and I wondered how many confirmed, and/or possible, kills he had in combat.
I tried to look it up, but couldn't find anything with Google.

This question appears to be off-topic because it is about the amount of kills an astronaut has.

This question is off-topic here, but it seems like it would be on-topic at History SE. Questions about historical events and famous people are explicitly on-topic there.

None, his combat unit was ground attack, not air to air, so it is unlikely he ever was involved in air to air combat.


text:
What percent of the world's airline operations are in the USA?
What percentage of the world's airline operations are in the USA?  And how does this number compare to other first world countries such as the UK and Canada?  How about places like China and India?  And finally smaller third-world countries?

What do you mean by "airline operations"? Number of flights? Number of passengers? Number of plane-miles? Number of passenger-miles? Is a flight from New York to London "in the USA"? Note also that the UK is a rather small country (a little smaller than Michigan) so there aren't many internal flights.

I was just thinking number of landings.  Or something "simple" like that.  So international flights would count for one country - since they should about average out.

It may be a bit hard to find this data for many different countries, especially if you want to separate takeoffs and landings.
According to this data for the US, there were 757.1 thousand flights in May 2014, including both domestic and international. Of those, 683.0 thousand were domestic, and 74.1 thousand were international.
Lots of information is available here. This indicates 8.3 million domestic and 1.4 million international flights in the US during 2013. This includes all carriers an all airports, including domestic all-cargo flights.
The best data for Europe I could find is only in passenger count (the more common statistic). It looks like you have to request access to get more detailed information.
However, this collection of data from Eurocontrol showed 9.548 million "IFR Flight Movements" in 2012. This would include all IFR flights, regardless of their type. A graph at the bottom shows figures for 2008. This shows a little over 8 million internal, and a little under 2 million arriving/departing. Since a domestic flight would be two movements (departure and arrival), this is 4 million and 2 million flights. As expected, Europe has a much higher percentage of international flights (about 1:2 international in Europe, versus about 1:9 in the US).
For domestic and international traffic in Australia, there are 702321 domestic flights and 162207 international flights in 2013, for a total of 864528 flights.
Data from the UK show that there are about 800000 domestic and 1000000 international flights per year.
You could also order a year of World Air Transport Statistics for only $799.

Do those IFR flight movements for Europe include general aviation and cargo or just scheduled commercial service? As far as the numbers listed for the U.S., that includes only scheduled commercial service on U.S.-based airlines. It does not include flight operations to or from the U.S. on foreign airlines, cargo traffic, or general aviation traffic. There were 9.17 million departures on U.S. airlines alone in the U.S. from July 2013 - June 2014 and this does not include cargo flights operated by the airlines.

Also, note the difference between 'movements' and 'departures.' Ideally, there are 2 movements per departure (a takeoff and a landing are each considered a 'movement.') In the case where there is only 1 movement per departure, that means the plane crashed, achieved orbit, or remained in flight indefinitely. :)

Also, note that the figure I linked above for the U.S. includes only revenue departures, not non-revenue flights (such as repositioning flights.)

tl;dr: Somewhere between 28.3% and 37.8%.
U.S.-flagged air carriers operating in the U.S. account for 28.3% worldwide scheduled operations while all air carrier traffic in North America accounts for 37.8% of worldwide traffic. Thus, the sum of all scheduled passenger operations in the U.S. is somewhere between those two figures.

Of the airports that participate in the Airports Council International statistics, 37.8% of aircraft movements for the year ending May 2014 were in the North America region, which does not include flights in the Latin America/Caribbean region. It appears that Mexico is included in the Latin America/Caribbean region, so it can be safely assumed that the vast majority, though not all, of the North American region traffic is in the U.S. Canada, of course, is also included in the North American region. The report linked above lists 24,571,773 movements in the North American region out of 64,949,170 for all reporting member airports worldwide.
The U.S. Department of Transportation maintains official statistics for U.S. air carriers. According to the U.S. DoT's Bureau of Transportation Statistics, in the year ending June 2014, there were 9,177,347 departures on U.S. airline passenger flights. Note that 'movements' in the ACI statistics above include both departures and arrivals as separate movements, so for purposes of comparison, the 9.18 million departures would equal 18.35 million movements. In order to get information for the exact same period measured by the ACI report (June 2013-May 2014 vs. the July 2013-June 2014 data in the DoT report,) we can subtract June 2014 departures and add June 2013 departures to the U.S. data. These numbers are given on the same DoT page linked above as 791,896 for June 2014 and 810,721 for June 2013. This yields 9,196,172 departures or 18.39 million movements, which accounts for 28.3% of ACI's worldwide movements for that period.
Note that flights on non-U.S.-based airlines arriving in or departing from the U.S. are not included in the U.S. DoT figures, nor are cargo flights or non-revenue flights (such as repositioning flights) included in their figures, so the actual percentage for the U.S. would be somewhat higher than that 28.3% figure, but lower than the 37.8% figure for all of North America.
Note also that the above links are unfortunately not permanently linked to the time series mentioned here. They will both continually be updated with the most up-to-date information, so, if you're viewing this answer significantly after it was posted, the exact figures given in those links will be newer than what I have included in this answer.

You can get a good feel for the numbers from this world map which shows total passengers carried around the world. Note: click on the "Points" option at the top left of the map to show the figures labelled on the map. US dominates when it comes to air travel, when domestic and international are included, with 743 million passengers in 2013. China came second with 353 million, then UK with 118 million.